Overrated: Sam and Max, Fahrenheit, Broken Sword, Longest Journey, Monkey Island 2, .
Underated: Loom, Quest for Glory (well, at least it doesn't seem to get discussed all that much around the place
), Pandora Directive. Oh, oh and Conquests of Camelot.
I would add The Last Express to underrated, but as far as it didn't sell well, it seems that most people that are aware of it know what a masterpiece it is.